The present invention relates to a spark plug applicable to a cogeneration purpose engine, and further relates to an adjusting method of this spark plug performed routinely to restore a discharge gap to an initial value in response to expansion of the discharging gap caused by spark exhaustion.
This kind of spark plug generally comprises a central electrode, an insulator provided around the central electrode, a metal fitting provided around the insulator, a grounding electrode having one end connected to the metal fitting and the other end located in opposed relationship with the central electrode, and noble metal tips attached on opposed surfaces of the central and grounding electrodes. The noble metal tips chiefly contain Ir (iridium). A discharging gap is thus formed between the noble metal tips attached on the opposed surfaces of the central and grounding electrodes.
The cogeneration purpose spark plugs are used at higher temperatures compared with the automotive vehicle spark plugs. Thus, the noble metal tips of the grounding and central electrodes are subjected to severe exhaustion. The discharging gap is relatively small. And, expansion of the discharging gap due to exhaustion of the noble metal tips gives bad influence to voltage characteristics of the plug.
Accordingly, this kind of spark plug requires an adjustment (generally referred to xe2x80x9cre-gappingxe2x80x9d) for restoring the discharge gap to an initial value which is performed routinely when the discharging gap has expanded to a certain extent due to exhaustion of the noble metal tips. The re-gapping adjustment is generally performed by deforming the distal end (i.e., free end) of the grounding electrode about the proximal end (i.e., stationary end) thereof so that the grounding electrode comes close to the central electrode. The discharging gap, having once expanded due to exhaustion of the noble metal tips, is thus reduced to the initial valve.
In this manner, this kind of spark plug is usually re-gapped once at an appropriate time after starting the spark operation from a brand-new condition. After the re-gapping adjustment, the noble metal tips are continuously worn away until it reaches the limit (i.e., exhaustion limit). The lifetime of the plug is generally represented by a period of time required for the noble metal tips to reach the unusable condition. When the lifetime ends, the worn-out plug needs to be replaced by a new one.
However, according to the inventors of this application, the following problems need to be solved to extend the lifetime of the conventional spark plugs. To obtain a satisfactory discharging area (i.e., to secure sufficient wear property), the conventional spark plugs employ a parallel discharging gap arrangement for the central electrode and the grounding electrode in such a manner that the noble metal tips of both electrodes are parallel to each other.
The noble metal tips are kept in parallel even when or after they are exhausted. However, performing the re-gapping adjustment forces the grounding electrode to decline or deform downward about its stationary end. Thus, after the re-gapping adjustment, the positional relationship between the noble metal tips attached on the grounding and central electrodes is no longer kept in parallel. The discharging gap, i.e., a spatial clearance between the opposed noble metal tips attached on the grounding and central electrodes, becomes narrow at one side closer to the free end of the grounding electrode and is wide at the opposite side closer to the stationary end of the grounding electrode.
The non-uniform discharging gap thus provided through the re-gapping adjustment brings bad influence to the noble metal tips. More specifically, regarding the degree of wear or exhaustion, it is faster at the one side of the noble metal tips closer to the free end of the grounding electrode and slower at the opposite side of the noble metal tips closer to the stationary end of the grounding electrode. In other words, after the re-gapping adjustment, wear or exhaustion of the noble metal tips of the spark plug advances non-uniformly in an inclined manner. In this case, the noble metal tips reach the exhaustion limit first at a local spot where the abrasion has advanced rapidly. Thus, the lifetime of the spark plug ends early even if the remaining portion of the noble metal tips are still usable.
As described above, the conventional spark plugs are disadvantageous in that the expensive noble metal tips are not effectively utilized after the re-gapping adjustment is performed. As a result, it was impossible to assure satisfactory wear properties. The lifetime of the conventional spark plugs was relatively short.
In view of the foregoing problems of the prior art, an object of the present invention is to provide a cogeneration purpose spark plug which essentially requires the re-gapping adjustment and more particularly to assure improved wear properties for the cogeneration purpose spark plug even after the re-gapping adjustment is performed, thereby extending the lifetime of the spark plug.
To accomplish the above and other related objects, the present invention provides a spark plug used for cogeneration purpose, comprising a central electrode, an insulator provided around the central electrode, a metal fitting provided around the insulator, a grounding electrode having one end connected to the metal fitting and the other end located in opposed relationship with the central electrode, and noble metal tips attached on opposed surfaces of the central electrode and the grounding electrode. A discharging gap is formed between the noble metal tip of the central electrode and the noble metal tip of the grounding electrode. Specifically, the spark plug of the present invention is characterized in that the discharging gap has a first gap G1 at one side closer to one end of the grounding electrode and a second gap G2 at the opposite side closer to the other end of the grounding electrode. The first gap G1 is shorter than the second gap G2. Hereinafter, this arrangement is referred to as an inclined discharging gap arrangement.
According to the present invention, the noble metal tips will be worn away locally and non-uniformly due to adoption of the inclined discharging gap arrangement until the re-gapping adjustment is performed. This will slightly shorten the lifetime of the spark plug in the initial stage where no re-gapping adjustment is required, compared with the conventional spark plug. However, the present invention makes it possible to locate the grounding electrode in parallel with the central electrode when the re-gapping adjustment is performed. Accordingly, after the re-gapping adjustment, the remaining noble metal tips are entirely worn out. In other words, almost all of the remaining noble metal tips can be effectively utilized for the final stage of the spark exhaustion. Thus, the present invention assures improved wear properties for the spark plug after the re-gapping adjustment and accordingly extends the lifetime of the spark plug.
According to the present invention, to employ the above-described inclined discharging gap arrangement, it is preferable that a first straight line S1 connecting both edges of the noble metal tip attached on the grounding electrode inclines by an angle xcex1 of 2xc2x0 to 10xc2x0 with a second straight line S2 connecting both edges of the noble metal tip attached on the central electrode, where the both edges of the noble metal tip of the grounding electrode and the both edges of the noble metal tip of the central electrode cooperatively define the first gap G1 and the second gap G2.
The angle xcex1 within a range of 2xc2x0 to 10xc2x0 assures the effect of the present invention. On the other hand, the angle xcex1 larger than 10xc2x0 will promote the local and non-uniform exhaustion of the noble metal tips before the re-gapping adjustment. Accordingly, the lifetime of the spark plug in the initial stage preceding the re-gapping adjustment will be so shortened that it cancels the effect of improved wear properties in the final stage succeeding the re-gapping adjustment.
According to the present invention, it is preferable that each of the noble metal tip of the grounding electrode and the noble metal tip of the central electrode is made of an Ir (iridium) alloy, e.g., 90Ir-10Rh, which has excellent wear properties. The lifetime of the spark plug can be extended.
According to the present invention, it is preferable that a minimum distance xcex94G of the discharging gap formed between the noble metal tip attached on the grounding electrode and the noble metal tip attached on the central electrode is within a range from 0.2 mm to 0.5 mm.
The discharging gap shorter than 0.2 mm will not bring satisfactory ignitability which is one of basic performances required for the spark plug. On the other hand, the discharging gap larger than 0.5 mm will not sufficiently assure the effect of the inclined discharging gap arrangement brought by the present invention. The lifetime of the spark plug will not be extended so much.
Furthermore, the present invention provides an adjusting method for a cogeneration purpose spark plug which comprises a metal fitting for mounting a central electrode via an insulator provided around the central electrode, a grounding electrode having one end connected to the metal fitting and the other end located in opposed relationship with the central electrode, and noble metal tips attached on opposed surfaces of the central electrode and the grounding electrode so that a discharging gap is formed between the noble metal tip of the central electrode and the noble metal tip of the grounding electrode, wherein the discharging gap is adjusted when the noble metal tips are exhausted to a certain extent.
More specifically, according to the adjusting method of the present invention, the noble metal tips of respective electrodes are disposed in such a manner that the discharging gap has a first gap G1 at one side closer to the one end of the grounding electrode and a second gap G2 at the opposite side closer to the other end of the grounding electrode, with the first gap G1 being shorter than the second gap G2, so that a first straight line S1 connecting both edges of the noble metal tip attached on the grounding electrode inclines by an angle xcex1 with respect to a second straight line S2 connecting both edges of the noble metal tip attached on the central electrode, where the both edges of the noble metal tip of the grounding electrode and the both edges of the noble metal tip of the central electrode cooperatively define the first gap G1 and the second gap G2. The discharging gap is adjusted to an adequate value when the discharging gap is increased due to exhaustion of the noble metal tips of both electrodes, by bringing the noble metal tip attached on the grounding electrode to a position closer to the noble metal tip attached on the central electrode so as to decrease the angle xcex1.
According to the adjusting method of the present invention, from the same reason set forth above, it becomes possible to assure improved wear properties for the spark plug after the re-gapping adjustment and accordingly extend the lifetime of the spark plug.
According to the adjusting method of the present invention, from the same reason set forth above, it is preferable that the angle xcex1 is within a range from 2xc2x0 to 10xc2x0.